1. Field of the Invention
The present invention relates to methods for manufacturing esters from acid and alcohol, and more particularly, to a method for manufacturing esters from acid and alcohol in a reactive distillation column and a system for carrying out the method.
2. Description of Related Art
Ethyl acetate, isopropyl acetate, and butyl acetate are important solvents in the industry for manufacturing plastic, grease, printing ink and so on. They can also be used for preparing synthetic flavor materials. There are a number of literatures related to methods for manufacturing ethyl acetates, which may be divided into the following categories in general:
(i) A pre-reactor is used for esterifying acetic acide to obtain ester products. The ester products are extracted and purified by a plurality of recovering columns and decanters. However, such a method is complicated to operate and costly to maintain as it involves a higher quantity of equipments and devices.
(ii) A reactive distillation column is used, which is primarily divided into a reaction section, a rectification section and a stripping section. Homogenous and heterogeneous catalysts are used to speed up the reaction.
(iii) A reactive distillation columns and a recovering column are used to obtain highly purified ethyl acetate from the bottom of the recovering column. However, the mixture of acid, alcohol and ester at the top of the recovering column has to be sent back to the reactive distillation column, and the aqueous alcohol solution at the bottom of the reactive distillation column has to be also processed.
Among the above conventional methods, the second one that a single reactive distillation column is used is most preferred, such that there are many discussions on literatures thereof. However, data disclosed by these literatures reveal that the method using a single reactive distillation column does not produce ethyl acetate with desired purity, making it fail to meet the industrial requirement. It is because the acid, alcohol and ester from an azeotropic mixture at the top of the reactive distillation column and the boiling point of the acetic acid at the bottom of the reactive distillation column is relatively high, making undesired products thus-obtained have to be reprocessed or eliminated.
In view of the above drawbacks, Taiwanese Patent No. TWI282746 discloses an improved system using a single reactive distillation column. The system is primarily made up of a reaction distillation column, a liquid decanter, and an ester recovering column, allowing the reactants to be fed into the reactive distillation column via the bottom plate thereof. The reactive distillation column is divided into a rectification section and a reactive section which is primarily filled with a solid catalyst and positioned between the upper portion and the bottom plate or between the middle portion and the bottom plate of the reactive distillation column. The liquid decanter then separates the azeotrope of alcohol-ester-water entered from the top plate of the reactive distillation column. Subsequently, a portion of the organic phase thus-separated is returned to the top plate of the reactive distillation column, while the rest of the organic phase is fed to the ester recovering column for extracting an ester product of a high purity, which can be obtained at the bottom of the ester recovering column.
However, problems in operation and design still exist in these reactive distillation mechanisms. In particular, in terms of filling the reactive distillation column with heterogeneous catalysts, there are often problems such as catalyst deterioration, and hardware design difficult for filling and refilling. Hence it is still required to have a method for making esters of a high purity above the industrial level as well as overcoming the drawback of the single reactive distillation column.